Dross occurs when soldering processes are performed. Dross consists of oxides of the metals of which the solder is composed, therefore usually tin oxide and lead oxide. Dross further comprises contaminants which come from the flux used or from other agents applied in the process. Dross has a density which is smaller than that of solder so that dross floats on the solder. Dross herein disrupts the soldering process.
It is known in the prior art to remove dross by regularly scooping it out of the solder bath and discharging it.
Research has shown however that dross is not only formed by metal oxides and contaminants of chemical agents but that dross is also formed by non-oxidized solder. During the oxidization process nuclei of solder are formed which are enveloped by oxides. Clusters of such units form the dross.
When the dross is scooped away not only are metal oxides and possible other contaminants therefore removed but also a considerable amount of usable solder. This results in the disposal of a large quantity of metals, of which a considerable part is still usable, which is undesirable from an environmental and economic viewpoint.
The object of the present invention is to provide a method and apparatus wherein the dross is separated into metal oxide and solder so that the drawbacks of the prior art are avoided.
This object is achieved by such a method wherein at least one knife extending substantially parallel to its direction of movement repeatedly cuts through the dross and the residue is removed from the solder.
This objective is also achieved by an apparatus comprising at least one knife which is movable along a path extending through a vessel, wherein the at least one knife is adapted to cut through the dross during the movement along the path.
Due to the above stated cutting operation the units of the dross formed by nuclei of solder enveloped by metal oxides are destroyed, whereby the solder is released and added to the solder already present in the bath and the released metal oxide will float on the solder. An important measure herein is that a cutting movement is performed. It is otherwise known to remove floating dross from solder by shifting the dross away with a scraping operation. A movement of a type of knife herein takes place transversely of the length direction thereof. Such a method and apparatus are known from the journal article xe2x80x9cRemoving dross from molten solderxe2x80x9d, NTIS TECH NOTES, 1990, p. 1100, Springfield, Va., USA.
In the method according to the invention a considerable volume and mass reduction of dross to metal oxide takes place. The metal oxide is generally in powder form and can then be easily removed.
It is pointed out here that after chemical reduction the metal oxide in question is suitable as pure raw material for the preparation of solder. This is an environmental and economic advantage. A further cleaning step will of course also have to be applied here, subject to the contamination.
According to an attractive embodiment the solder is transported to a separating vessel before the cutting operation takes place and the resulting solder is fed back to the vessel.
According to this embodiment this vessel is a separating vessel and this latter is connected to a processing vessel by supply means for carrying solder provided with dross from the processing vessel to the separating vessel and by discharge means for feeding solder back from the separating vessel to the processing vessel, wherein the supply means debouch at a higher level in the separating vessel than the discharge means.
As a consequence of these measures it is possible to carry out in normal manner the operations to be performed in the processing vessel, such as soldering of objects, while the separating operation can take place independently. The separating vessel can therefore be optimized in terms of dimensioning and the like for the operations to be performed therein.
Other attractive embodiments are stated in the sub-claims.